Shared preferences/addons across different machines (e.g. Dropbox)
It seems like this thread should help. Copying startup.blend (this contains your startup scene) and userpref.blend (this contains your user preferences) to Dropbox and syncing them should have the result you're after. You would have to manually copy and paste the files to and from the Dropbox folder on each machine each time you changed them, however, so it won't be an automatic update.

ex. Working at home, make changes to User Preferences, copy userpref.blend to Dropbox, go to work, move userpref.blend from Dropbox to Blender folder. You would also have to do the same with the addons you installed.